1. Technical Field
The present invention relates to a capacitance type sensor, an acoustic sensor, and a microphone. More specifically, the present invention relates to a capacitance type sensor of a capacitor structure including a vibration electrode plate (diaphragm) and a fixed electrode plate. In addition, the present invention relates to an acoustic sensor which converts acoustic vibration to an electric signal to output the electric signal, and a microphone using the acoustic sensor. In particular, the present invention relates to a capacitance type sensor and an acoustic sensor of very small size manufactured by using MEMS (Micro Electro Mechanical System) technique.
2. Related Art
As small microphones incorporated into mobile phones, electret condenser microphones have been widely used. However, the electret condenser microphones are heat-sensitive, and are inferior to MEMS microphones in point of coping with digitization, miniaturization, advanced and more functions, and electric power saving. Therefore, at present, the MEMS microphones are becoming widespread.
The MEMS microphones have an acoustic sensor (acoustic transducer) which detects acoustic vibration to convert it to an electric signal (detection signal), a drive circuit which applies a voltage to the acoustic sensor, and a signal processing circuit which subjects the detection signal from the acoustic sensor to signal processing, such as amplification, to output the processed signal to the outside. The acoustic sensor used for the MEMS microphones is a capacitance type acoustic sensor manufactured by using MEMS technique. In addition, the drive circuit and the signal processing circuit are integrally manufactured as an ASIC (Application Specific Integrated Circuit) by using a semiconductor manufacturing technique.
In recent years, the microphones are required to detect sounds ranging from low sound pressure to high sound pressure at high sensitivity. The maximum input sound pressure of the microphones is typically limited according to a total harmonic distortion. This is because harmonic distortion is caused in an output signal when the microphones detect a sound having a high sound pressure, resulting in deteriorating sound quality and accuracy. Therefore, when the total harmonic distortion can be reduced, the maximum input sound pressure can be higher to widen the detection sound pressure range (hereinafter, dynamic range) of the microphones.
However, in the typical microphones, a trade-off relation exists between improvement in acoustic vibration detection sensitivity and reduction in total harmonic distortion. Consequently, in the high-sensitivity microphone which can detect a sound having a low sound volume (low sound pressure), the total harmonic distortion in an output signal becomes higher at the time of entering of a high-volume sound, resulting in limiting the maximum detection sound pressure. This is because the output signal of the high-sensitivity microphone becomes greater to be likely to cause harmonic distortion. On the contrary, when the maximum detection sound pressure is increased by reducing the harmonic distortion in an output signal, the sensitivity of the microphone becomes lower to make detection of a sound having a low sound volume at high quality difficult. As a result, the typical microphones are difficult to have a wide dynamic range from low sound volume (low sound pressure) to high sound volume (high sound pressure).
Under such a technical background, to have the wide dynamic range, microphones using a plurality of acoustic sensors having different detection sensitivities have been studied. Such microphones are disclosed in e.g., Patent Documents 1 to 4.
Patent Documents 1 and 2 disclose the microphone which is provided with a plurality of acoustic sensors and switches or combines a plurality of signals from the acoustic sensors according to sound pressure. Such a microphone has a detectable sound pressure level (SPL) of approximately 30 dB to 140 dB by switching a high-sensitivity acoustic sensor having a detectable sound pressure level of approximately 30 dB to 115 dB and a low-sensitivity acoustic sensor having a detectable sound pressure level of approximately 60 dB to 140 dB. In addition, Patent Documents 3 and 4 disclose the microphone which has a plurality of independent acoustic sensors formed over one chip.
FIG. 1A shows the relation between the total harmonic distortion and the sound pressure in the high-sensitivity acoustic sensor of Patent Document 1. FIG. 1B shows the relation between the total harmonic distortion and the sound pressure in the low-sensitivity acoustic sensor of Patent Document 1. FIG. 2 shows the relation between the average displacement amount of diaphragms and the sound pressure in the high-sensitivity acoustic sensor and the low-sensitivity acoustic sensor of Patent Document 1. When the allowed total harmonic distortion is 20%, the maximum detection sound pressure of the high-sensitivity acoustic sensor is approximately 115 dB. The high-sensitivity acoustic sensor has a minimum detection sound pressure of approximately 30 dB because when the sound pressure is lower than approximately 30 dB, the S/N ratio is deteriorated. Therefore, as shown in FIG. 1A, the dynamic range of the high-sensitivity acoustic sensor is approximately 30 dB to 115 dB. Likewise, when the allowed total harmonic distortion is 20%, the maximum detection sound pressure of the low-sensitivity acoustic sensor is approximately 140 dB. The diaphragm of the low-sensitivity acoustic sensor has a smaller area than the high-sensitivity acoustic sensor, and as shown in FIG. 2, has a smaller average displacement amount than the high-sensitivity acoustic sensor. Therefore, the minimum detection sound pressure of the low-sensitivity acoustic sensor is higher than the minimum detection sound pressure of the high-sensitivity acoustic sensor, and is approximately 60 dB. As a result, as shown in FIG. 1B, the dynamic range of the low-sensitivity acoustic sensor is approximately 60 dB to 140 dB. When the high-sensitivity acoustic sensor is combined with the low-sensitivity acoustic sensor, as shown in FIG. 1C, the detectable sound pressure range becomes wider and is approximately 30 dB to 140 dB.
The total harmonic distortion is defined as follows. The waveform indicated by the solid line in FIG. 3A is a basic sine waveform at frequency f1. When the basic sine waveform system is Fourier transformed, a spectrum component appears only in the position of frequency f1. Assume that the basic sine waveform in FIG. 3A is distorted due to some cause like the waveform indicated by the dashed line in FIG. 3A. When the distortion waveform is Fourier transformed, the frequency spectrum in FIG. 3B is obtained. That is, assume that the distortion waveform has FFT intensities (fast Fourier transformation intensities) of V1, V2, . . . , V5 at frequencies f1, f2, . . . , f5, respectively. At this time, total harmonic distortion THD of the distortion waveform is defined by the following equation 1.
                    THD        =                                                            V                ⁢                                                                  ⁢                                  2                  2                                            +                              V                ⁢                                                                  ⁢                                  3                  2                                            +                              V                ⁢                                                                  ⁢                                  4                  2                                            +                              V                ⁢                                                                  ⁢                                  5                  2                                                                          V            ⁢                                                  ⁢            1                                              (                  Equation          ⁢                                          ⁢          1                )            